Brushless direct current (BLDC) motors are used in industries such as appliances, automotive, aerospace, consumer, medical, industrial automation equipment and instrumentation. BLDC motors do not use brushes for commutation, instead, electronic commutation is used. BLDC motors have advantages over brushed DC motors and induction motors such as: better speed versus torque characteristics, high dynamic response, high efficiency, long operating life, longer time intervals between service, substantially noiseless operation, and higher speed ranges. A more detailed synopsis of BLDC motors may be found in Microchip Application Note AN857, entitled “Brushless DC Motor Control Made Easy;” and Microchip Application Note AN885, entitled “Brushless DC (BLDC) Motor Fundamentals;” both at www.microchip.com, and wherein both are hereby incorporated by reference herein for all purposes.
BLDC motor control provides three things: (1) pulse width modulation (PWM) drive voltages to control the motor speed, (2) a mechanism to commutate the stator of the BLDC motor, and (3) a way to estimate the rotor position of the BLDC motor. PWM may be used to provide a variable voltage to the stator windings of the BLDC motor. The effective voltage provided thereto is proportional to the PWM duty cycle. The inductances of the stator coils act as low pass filters to smooth out the PWM pulses to substantially direct current (DC) voltages. When properly commutated, the torque-speed characteristics of a BLDC motor are substantially identical to a DC motor. The variable voltage controls the speed of the motor and the available torque.
A three-phase BLDC motor completes an electrical cycle, i.e., 360 electrical degrees of rotation, in six steps at 60 electrical degrees per step. Synchronously at every 60 electrical degrees, winding phase current switching is updated (commutation). However, one electrical cycle may not correspond to one mechanical revolution (360 mechanical degrees) of the motor rotor. The number of electrical cycles to be repeated to complete one mechanical revolution depends upon the number of rotor pole pairs.
BLDC motors are not self-commutating and therefore are more complicated to control. BLDC motor control requires knowledge of the motor rotor position and a mechanism to commutate the BLDC motor stator windings. For closed-loop speed control of a BLDC motor there are two additional requirements, measurement of rotational speed and a pulse width modulation (PWM) drive signal to control the motor speed and power therefrom.
To sense the rotor position of the BLDC motor, Hall Effect sensors may be used to provide absolute rotor position sensing. However, Hall Effect sensors increase the cost and complexity of a BLDC motor. Sensorless BLDC control eliminates the need for Hall Effect sensors by monitoring the back electromotive force (BEMF) voltages at each phase (A-B-C) of the motor to determine drive commutation. The drive commutation is synchronized with the motor when the BEMF of the un-driven phase crosses one-half of the motor supply voltage in the middle of the commutation period. This is referred to as “zero-crossing” where the BEMF varies above and below the zero-crossing voltage over each electrical cycle. Zero-crossing can only be detected on the un-driven phase when the drive voltage is being applied to the driven phases. So detecting a change of the BEMF on the un-driven phase from less than to greater than one-half of the motor supply voltage may be used during application of the drive voltage to the two driven phases for a three phase BLDC motor.
One of the simplest methods of control for a BLDC motor is trapezoidal commutation. Switching (commutation), e.g., using power transistors, energizes the appropriate two stator windings of a three phase BLDC motor depending upon the rotor position. The third winding remains disconnected from the power source. During rotation of the rotor currents, two of the stator winding are equal in magnitude and the third unconnected stator winding current is zero (for a WYE connected stator windings). With a three phase BLDC motor there are only six different space vector directions and as the rotor turns, the current through two of the stator windings (WYE connected stator windings) is electrically switched (commutated) every 60 degrees of electrical rotation so that the current space vector is always within the nearest 30 degrees of the quadrature direction. The current waveform for each winding is therefore a staircase from zero, to positive current, to zero, and then to negative current. This produces a current space vector that approximates smooth rotation as it steps among six distinct directions as the rotor turns. The trapezoidal-current driven BLDC motors are popular because of the simplicity of control but suffer from higher torque ripple and lower efficiency than sinusoidal drive.
Sinusoidal commutation drives the three stator windings of the BLDC motor with three currents that vary smoothly as the rotor turns. The relative phases of these currents are chosen, e.g., 120 degrees apart, so that they provide for a smoothly rotating current space vector that is always in the quadrature direction with respect to the rotor and has constant magnitude. This eliminates the torque ripple and commutation spikes associated with trapezoidal commutation. However, sinusoidal commutation drive systems are more complex and expensive than trapezoidal commutation drive systems.